Hydraulic conductivity monitoring to initiate tissue division

ABSTRACT

A method for performing an electrosurgical procedure at a surgical site on a patient includes continually sensing electrical and physical properties proximate the surgical site that includes acquiring readings of tissue electrical impedance with respect to time at the surgical site; identifying the minima and maxima of the impedance readings with respect to time; and correlating the minima and/or the maxima of the impedance readings with hydration level and/or hydraulic conductivity in the tissue at the surgical site. The method also includes controlling the application of electrosurgical energy to the surgical site to vary energy delivery based on the step of correlating the minima and/or the maxima of the impedance readings with the hydration level/or and the hydraulic conductivity in the tissue at the surgical site. The process may be an ablation process.

BACKGROUND

1. Technical Field

This application relates to electrosurgical surgery and, in particular,to control systems for electrosurgical generators and to methods ofdetermining tissue moisture content during use of electrosurgicalinstruments.

2. Description of Related Art

Electrosurgical generators are employed by surgeons in conjunction withan electrosurgical instrument to cut, coagulate, desiccate and/or sealpatient tissue. High frequency electrical energy, e.g., radio frequency(RF) energy, is produced by the electrosurgical generator and applied tothe tissue by the electrosurgical tool. Both monopolar and bipolarconfigurations are commonly used during electrosurgical procedures.

Electrosurgical techniques and instruments can be used to coagulatesmall diameter blood vessels or to seal large diameter vessels ortissue, e.g., soft tissue structures, such as lung, brain and intestine.A surgeon can either cauterize, coagulate/desiccate and/or simply reduceor slow bleeding, by controlling the intensity, frequency and durationof the electrosurgical energy applied between the electrodes and throughthe tissue. For the purposes herein, the term “cauterization” is definedas the use of heat to destroy tissue (also called “diathermy” or“electrodiathermy”). The term “coagulation” is defined as a process ofdesiccating tissue wherein the tissue cells are ruptured and dried.“Vessel sealing” is defined as the process of liquefying the collagenand elastin in the tissue so that it reforms into a fused mass withsignificantly-reduced demarcation between the opposing tissue structures(opposing walls of the lumen). Coagulation of small vessels is usuallysufficient to permanently close them. Larger vessels or tissue need tobe sealed to assure permanent closure.

In order to achieve one of the above desired surgical effects withoutcausing unwanted charring of tissue at the surgical site or causingcollateral damage to adjacent tissue, e.g., thermal spread, it isnecessary to control the output from the electrosurgical generator,e.g., power, waveform, voltage, current, pulse rate, etc.

It is known that measuring the electrical impedance and change thereofacross the tissue at the surgical site provides a good indication of thestate of desiccation or drying of the tissue, e.g., as the tissue driesor loses moisture, the impedance across the tissue rises. Thisobservation has been utilized in some electrosurgical generators toregulate the electrosurgical power based on a measurement of tissueimpedance. For example, commonly owned U.S. Pat. No. 6,210,403 relatesto a system and method for automatically measuring the tissue impedanceand altering the output of the electrosurgical generator based on themeasured impedance across the tissue. The entire contents of this patentis hereby incorporated by reference herein.

It has been determined that the particular waveform of electrosurgicalenergy can be tailored to enhance a desired surgical effect, e.g.,cutting, coagulation, sealing, blend, etc. For example, the “cutting”mode typically entails generating an uninterrupted sinusoidal waveformin the frequency range of 100 kHz to 4 MHz with a crest factor in therange of 1.4 to 2.0. The “blend” mode typically entails generating anuninterrupted cut waveform with a duty cycle in the range of 25% to 75%and a crest factor in the range of 2.0 to 5.0. The “coagulate” modetypically entails generating an uninterrupted waveform with a duty cycleof approximately 10% or less and a crest factor in the range of 5.0 to12.0. In order to effectively and consistently seal vessels or tissue, apulse-like waveform may be employed. Energy may be supplied in acontinuous fashion to seal vessels in tissue if the energy input/outputis responsive to tissue hydration/volume through feedback control.Delivery of the electrosurgical energy in pulses allows the tissue tocool down and also allows some moisture to return to the tissue betweenpulses which are both known to enhance the sealing process.

It is further known to clamp or clip excess voltage output from theelectrosurgical generator by the use of avalanche devices, such asdiodes, zener diodes and transorbs, resulting in absorption anddissipation of excess energy in the form of heat.

Commonly owned U.S. Pat. No. 6,398,779 discloses a sensor which measuresthe initial tissue impedance with a calibrating pulse which, in turn,sets various electrical parameters based on a look-up table stored in acomputer database. The transient pulse width associated with each pulsemeasured during activation is used to set the duty cycle and amplitudeof the next pulse. Generation of electrosurgical power is automaticallyterminated based on a predetermined value of the tissue impedance acrossthe tissue.

Commonly owned U.S. Patent Application Publication US 2007/0038209 A1 byBuysse et al., “METHOD AND SYSTEM FOR CONTROLLING OUTPUT OF RF MEDICALGENERATOR”, the entire contents of which is incorporated by referenceherein, discloses a sensor module for sensing (measuring) tissuemoisture (which is often indicative of tissue type) and generating amoisture content value and/or determining tissue type. Moisture contentis determined from tissue compliance data, e.g., measurement of tissuedisplacement divided by an applied force, or optical clarity. Theadditional sensor module may include an infrared or optical sensor forsensing (measuring) light or energy generated by a source, such as aninfrared or other light source, which is transmitted through orreflected from the tissue, where the sensed value is indicative oftissue moisture content and/or tissue type of tissue proximate asurgical site. An initial tissue moisture content value and/or tissuetype may be provided to a control module as a pre-surgical parameter.Sensed real time moisture content values and/or changes in moisturecontent over time (Δ(difference) moisture content/Δ(difference) time)may further be provided to the control module during the surgicalprocedure, where the control module modulates the electrical surgicaloutput in accordance with the sensed real time moisture content valuesand/or changes in moisture content values over time.

As can be appreciated therefore, since electrosurgical devices areenergy based by nature, electrosurgical devices, includingradiofrequency (RF), ablation, and similar energy based medical deviceshave sensors to monitor energy delivery. Such sensors include voltageand current measuring devices. Voltage and current measurement aredirect indicators of safety parameters of the electrosurgical instrumentand are indirect indicators of tissue impedance and correspondingly oftissue hydration. Tissue hydration affects the degree of cooling andheat transfer available through the local water content of the tissue toprevent overdessication.

SUMMARY

The present disclosure advances the state of the art of controlling theenergy input from an electrosurgical device by adjusting the hydrationlevel and direction of water motility to significantly increase thelikelihood of prevention of overdessication and to initiate tissuedivision.

More particularly, the present disclosure relates to a method forperforming an electrosurgical procedure at a surgical site on a patient.The method includes the step of: continually sensing electrical andphysical properties proximate the surgical site. The step of continuallysensing properties includes acquiring readings of tissue electricalimpedance with respect to time at the surgical site; identifying theminima and maxima of the impedance readings with respect to time; andcorrelating at least one of the minima and the maxima of the impedancereadings with at least one of hydration level and hydraulic conductivityin the tissue at the surgical site. The method also includes controllingthe application of electrosurgical energy to the surgical site to varyenergy delivery based on the step of correlating at least one of theminima and the maxima of the impedance readings with at least one of thehydration level and the hydraulic conductivity in the tissue at thesurgical site.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will be described herein below with reference to thedrawings wherein:

FIG. 1 is a schematic diagram of a closed-loop control system for usewith an electrosurgical generator according to the present disclosure;

FIG. 2 is a schematic diagram of a sensor module for use with theclosed-loop control system of FIG. 1;

FIG. 3 is a flowchart illustrating a method of operation of theclosed-loop control system according to the present disclosure;

FIG. 4 is a block diagram of a dual loop control system in accordancewith another embodiment of the present disclosure; and

FIG. 5 is a graphical plot of impedance versus time resulting from atissue ablation process and highlighting times of occurrence of lowmotility of water in the tissue and of high motility of water in thetissue according to one embodiment of the present disclosure;

DETAILED DESCRIPTION

As described in the background, electrical and thermal conductivityproperties of tissue that are exhibited during the application ofelectrosurgical energy to tissue have been well elucidated in the field.

The present disclosure advances the state of the art of the applicationof electrosurgical energy to tissue by introducing the concept ofhydraulic conductivity (also referred to as water motility) andillustrating how impedance information can be interpreted to measurethis parameter.

Hydraulic conductivity, symbolically represented herein as “K”, is aproperty of vascular or capillary beds that, in contrast to the simplemeasure of moisture content of tissue, describes the ease with whichwater can move through pore spaces or fractures. “K” depends on theintrinsic permeability of the tissue and on the degree of saturation.One application of hydraulic conductivity “K” is as a factor in theStarling equation, which enables calculation of flow across walls ofcapillaries and introduces the idea of a reflection coefficient.

The Starling equation is defined as follows:J _(v) =K _(f)([P _(c) −P _(i)]−σ[π_(c)−π_(i)])  (1)where:

-   -   ([P_(c)−P_(i)]−σ[π_(c)−π_(i)]) is the net driving force,    -   K_(f) is the proportionality constant, and    -   J_(v) is the net fluid movement between compartments.        According to Starling's equation, the movement of fluid depends        on six variables:    -   1. Capillary hydrostatic pressure (P_(c))    -   2. Interstitial hydrostatic pressure (P_(i))    -   3. Capillary oncotic pressure (π_(c))    -   4. Interstitial oncotic pressure (π_(i))    -   5. Filtration coefficient (K_(f))    -   6. Reflection coefficient (σ)

(Oncotic pressure, or colloid osmotic pressure, is a form of osmoticpressure exerted by proteins in blood plasma that usually tends to pullwater into the circulatory system).

By convention, outward force is defined as positive, and inward force isdefined as negative. The solution to the equation is known as the netfiltration or net fluid movement (J_(v)). If positive, fluid will tendto leave the capillary (filtration). If negative, fluid will tend toenter the capillary (absorption). This equation has a number ofimportant physiologic implications, especially when pathologic processesgrossly alter one or more of the variables.

More generally, hydraulic conductivity is the proportionality constantin Darcy's law, which relates the amount of water which will flowthrough a unit cross-sectional area of aquifer under a unit gradient ofhydraulic head. It is analogous to the thermal conductivity of materialsin heat conduction, or the inverse of resistivity in electricalcircuits. The hydraulic conductivity (“K”—the English letter “kay”) isspecific to the flow of a certain fluid (typically water, sometimes oilor air); intrinsic permeability (“κ”—the Greek letter “kappa”) is aparameter of a porous media which is independent of the fluid. Thismeans that, for example, “K” will increase if the water in a porousmedium is heated (reducing the viscosity of the water), but “κ” willremain constant. “K” and “κ” are related through the following equation:K=(κγ)/μ  (2)where

“K” is the hydraulic conductivity [LT⁻¹ or m s⁻¹];

“κ” is the intrinsic permeability of the material [L² or m²];

“γ” is the specific weight of water [ML⁻²T⁻² or N m⁻³], and;

“μ” is the dynamic viscosity of water [ML⁻¹T⁻¹ or kg m⁻¹ s⁻¹].

and where

“L”=length or meters (m);

“T”=time or seconds (s); and

“M”=mass or kilograms (kg)

The reflection coefficient relates to the permeability of the tissue. Ifthe permeability of the particular sample of tissue is high as comparedto another sample of tissue, the reflection coefficient of theparticular sample of tissue is lower than the reflection coefficient ofthe sample of tissue that is less permeable. This is because more waterwill pass through the more permeable sample of tissue and less will bereflected back (analogous to the flow of water through a net).Conversely, when the permeability is low, more water will be reflectedback and thus the reflection coefficient is comparatively higher(analogous to the flow of water hitting a solid wall rather than a net).Since a solid or a fluid in motion, e.g., a steam pocket, will create aninstantaneous dynamic load on a stationary object in its path, thedynamic load on the tissue will be higher when the reflectioncoefficient of the tissue if higher. As the dynamic load increases, thegreater the susceptibility of the particular sample of tissue to tissuedivision and, when desired, to tissue destruction.

All-in-all, it is easier to explain Darcy's equation using the“hydraulic conductivity” term. It should be mentioned that the StarlingEquation (1) accounts for both hydraulic and osmotic pressures that arenot specified in the more generalized Darcy Equation (2).

As is known in the art, the water contained within the patient tissue isthe solvent that separates and mobilizes the electrically charged ionsin the solvent to create an ionic solution. The ionic solution ischaracterized by the current density or ion flow that creates thedesired surgical effect. Without water and the dissolved or solute ionstherein, no, or extremely limited electrical current, can be conveyedthrough the tissue because the tissue would then be characterized by ahigh electrical impedance. The parameter of hydraulic conductivity “K”provides a metric for how readily water can travel through the tissuewhen energy is applied. When temperatures cause the tissue water toboil, the gas bubbles that form displace liquid water only at the ratethat hydraulic conductivity allows. Water is displaced easily ifhydraulic conductivity is high; so water can move away from the boilingsite easily and can also rehydrate previously dehydrated tissue veryrapidly.

If hydraulic conductivity is low then the pressure of water phasetransition may become explosively high. The pressure may create steamjets that find paths of least resistance to areas of lower pressure.These steam jets may create paths along the tissue-device boundary andvent to remote cavities or the atmosphere. If these “steam jets” arecreated in a controlled manner, then work, such as tissue division, canoccur. The application of this tissue division as a function ofdifferent states of hydraulic conductivity is discussed below withrespect to FIG. 5. As described herein, the tissue division occursdirectly or indirectly due to the application of electrical energy, incontrast to tissue division that occurs mechanically, such as via aknife blade in an electrosurgical instrument.

For example, it is desired for liver trans-sections to create bloodlesszones to remove a lobe in the liver without blood loss and without aseparate cutting step. In conventional ablation processes, althoughheated and containing coagulated blood, the tissue remains substantiallyintact. In the embodiments of the present disclosure, the surgeon cancause tissue division by the process of measuring the impedance levelsand applying electrosurgical energy to the tissue, effectively cuttingthe tissue due to the electrical energy of the ablation process itselfwithout requiring a separate step of mechanically cutting the tissue.

Turning the discussion now to the detailed description of theembodiments of the present disclosure, reference should be made to thedrawings where like reference numerals refer to similar elementsthroughout the various figures, Referring to FIG. 1, there is shown aschematic diagram of one embodiment of the presently disclosed closedloop control system 100 for use with an electrosurgical generator 101.Control system 100 includes a control module 102, user interface 108 andsensor module 110. The control module 102 is operatively connected tothe electrosurgical generator 101. The electrosurgical generator 101 mayinclude electrosurgical energy output stage 104 and a power supply 106,where the output stage 104 receives power from the power supply 106 anddelivers RF energy to a patient 112 via at least one electrode (notshown). As can be appreciated, one or more electrodes may be used withthe electrosurgical instrument for performing monopolar or bipolarsurgery.

The sensor module 110 senses various electrical and physical parametersor properties at the operating site and communicates with the controlmodule 102 to regulate the electrosurgical output from the output stage104. The sensor module 110 may be configured to measure or “sense”various electrical or electromechanical conditions at the operating sitesuch as: tissue impedance, changes in tissue impedance, maxima andminima of tissue impedance, tissue temperature, changes in tissuetemperature, leakage current, applied voltage and applied current. Thesensor module 110 may measure one or more of these conditionscontinuously or in “real time” such that the control module 102 cancontinually modulate the electrosurgical output according to a specificpurpose or desired surgical intent. More particularly, analog signalsprovided by the sensor module 110 are converted to digital signals viaan analog-to-digital converter (ADC) 114, which in turn are provided tothe control module 102.

The control module 102, thereafter, regulates the power supply 106and/or the output stage 104 according to the information obtained fromthe sensor module 110. The user interface 108 is electrically connectedto the control module 102 to allow the user to control variousparameters of the electrosurgical energy output to the patient 114during surgery to manually set, regulate and/or control one or moreelectrical parameters of the delivered RF energy, such as voltage,current, power, frequency, amplified, and/or pulse parameters, e.g.,pulse width, duty cycle, crest factor, and/or repetition rate dependingupon a particular purpose or to change surgical intent.

The control module 102 includes at least one microprocessor capable ofexecuting software instructions for processing data received by the userinterface 108 and the sensor module 110 for outputting control signalsto the output stage 104 and/or the power supply 106, accordingly. Thesoftware instructions executable by the control module are stored in aninternal memory in the control module 102, an internal or externalmemory bank accessible by the control module 102 and/or an externalmemory, e.g., an external hard drive, floppy diskette, CD-ROM, etc.Control signals from the control module 102 to the electrosurgicalgenerator 101 may be converted to analog signals by a digital-to-analogconverter (DAC) 116.

In particular, for the purposes of highlighting the difference betweenthe present disclosure and the prior disclosure of U.S. PatentApplication Publication US 2007/0038209 A1, the control module 102further includes a dedicated tissue conductivity/impedance graphical ornumerical hydration analyzer and controller module 102′ that performs agraphical and/or numerical analysis of tissue conductivity/impedancedata as sensed by the sensor module 110. As explained in more detailbelow with respect to FIG. 5, the graphical and/or numerical analysis isspecifically directed to identifying specific portions of the maxima andminima of the tissue conductivity/impedance data that correlate tohydration level and direction of water motility. Alternatively, thecontrol module 102 is programmed with machine-readable code to performthe aforementioned graphical and/or numerical analysis and control.

The power supply 106 is a high voltage DC power supply for producingelectrosurgical current, e.g., radiofrequency (RF) current. Signalsreceived from the control module 102 or hydration analyzer andcontroller module 102′ control the magnitude of the voltage and currentoutput by the DC power supply based on the data readings for maxima andminima of the tissue conductivity/impedance. The output stage 104receives the output current from the DC power supply and generates oneor more pulses via a waveform generator (not shown). As can beappreciated, the pulse parameters, such as pulse width, duty cycle,crest factor and repetition rate are regulated in response to thesignals received from the control module 102 based on those portions ofthe data readings for maxima and minima of the tissueconductivity/impedance that specifically relate to hydration level anddirection of water motility. Alternatively, the power supply 106 may bean AC power supply, and the output stage 104 may vary the waveform ofthe signal received from power supply 106 to achieve a desired waveform.

As mentioned above, the user interface 108 may be local to or remotefrom the control module 102 or hydration analyzer and controller module102′. A user may enter data such as the type of electrosurgicalinstrument being used, the type of electrosurgical procedure to beperformed, and/or the tissue type upon which the electrosurgicalprocedure is being performed. The closed loop control system 100, inparticular the sensor module 110, may include, in addition to thesensors that detect voltage and current to determine the maxima andminima of tissue impedance, one or more smart sensors which providefeedback to the surgeon relating to one or more of these physicalparameters. Furthermore, the user may enter commands, such as a targeteffective voltage, current or power level to be maintained, or a targetresponse e.g., change in regulation of the power supply 106 and/oroutput stage 104, to changes in sensed values, such as an effectivechange in voltage, current and/or power level as a function particularlyof those portions of the data readings for maxima and minima of thetissue conductivity/impedance that specifically relate to hydrationlevel and direction of water motility. The user may also enter commandsfor controlling electrical parameters of the RF energy, delivered by theelectrosurgical generator 101, as described above. Default values may beprovided for the above target levels and target responses.

The sensor module 110 includes a plurality of sensors (not shown)strategically located for sensing various properties or conditions at orproximate points “A” and “B”. Sensors positioned at or proximate point“A” (hereinafter referred to as at point “A”) sense properties and/orparameters of electrosurgical output from output stage 104, and/orproperties, parameters or conditions prior to surgical effect of thecurrently administered electrosurgical energy during the surgicalprocedure. For example, sensors positioned at point “A” may be providedwith or attached proximate the generator 101.

Sensors positioned at or proximate point “B” (hereinafter referred to asat point “B”) sense parameters, properties and/or conditions at oracross the operating site prior to the surgical procedure and/or inresponse to surgical effect during the surgical procedure. One or moreof these sensors may be included with the electrosurgical instrument,(e.g., on one end effector or opposing end effectors) or attachedproximate the operating site. For example, optical sensors, proximitysensors, temperature sensors may be used to detect certain tissuecharacteristics, and electrical sensors may be employed to sense otherparameters of the tissue or operating effects. It is noteworthy thatpoint “A” may be located proximate the surgical site “B” at a locationwhere the signals outputted by the generator 101 are propagated beforethey are applied or approximately when they are applied to the surgicalsite “B”.

The sensors are provided with leads or wireless means for transmittinginformation to the control module, where the information is provideddirectly to the control module 102 and/or to the hydration analyzer andcontroller module 102′, and/or provided to the control module 102 and/orto the hydration analyzer and controller module 102′ via the sensormodule 110 and/or the ADC 114. The sensor module 110 may include meansfor receiving information from multiple sensors, and providing theinformation and the source of the information (e.g., the particularsensor providing the information) to the control module 102 and/or tothe hydration analyzer and controller module 102′.

Related prior art methods and systems for controlling the output ofelectrosurgical generators are described in commonly owned U.S. PatentApplication Ser. No. 11,585,506 filed on Oct. 24, 2006 by Buysse et al.,now previously mentioned U.S. Patent Application Publication2007/0038209 A1, entitled “METHOD AND SYSTEM FOR CONTROLLING OUTPUT OFRF MEDICAL GENERATOR”, which is a divisional of U.S. patent applicationSer. No. 10/427,832 filed on May 1, 2003 and now U.S. Pat. No. 7,137,980issued on Nov. 21, 2006 to Buysse et al., “METHOD AND SYSTEM FORCONTROLLING OUTPUT OF RF MEDICAL GENERATOR”; U.S. patent applicationSer. No. 10/073,761, filed on Feb. 11, 2002, by Wham et al., now U.S.Patent Application Publication 2003/0004510 A1, entitled “VESSEL SEALINGSYSTEM”; and U.S. patent application Ser. No. 09/408,944, now previouslymentioned U.S. Pat. No. 6,398,779, filed on Sep. 30, 1999 by Buysse etal., entitled “VESSEL SEALING SYSTEM”, which claims the benefit of thepriority date for U.S. provisional application No. 60/105,417, filed onOct. 23, 1998, the entire contents of all of these applications arehereby incorporated by reference herein in their entirety.

With reference to FIG. 2, in addition to FIG. 1, the inner-workingcomponents of the sensor module 110 are shown in greater detail. Moreparticularly, the sensor module 110 may include a real-time voltagesensing system 202 and a real-time current sensing system 204 forsensing real-time values for applied voltage and current at the surgicalsite “B”. The sensor module 110 also may include a real-time voltagesensing system 206 and a real-time current sensing system 208 forsensing real-time values of signals returned from the patient at a point“A”. An RMS (root-mean-square) voltage sensing system 210 and an RMScurrent sensing system 212 are also included for sensing and derivingRMS values for applied voltage and current at the surgical site “B”, andan RMS voltage sensing system 214 and an RMS current sensing system 216are included for sensing and deriving RMS values of signals at point“A”. A temperature sensing system 218 may be included for sensing tissuetemperature at the surgical site “B”. Real-time and RMS current andvoltage sensing systems are known in the art. The sensor module 110 mayfurther include sensors (not shown) for sensing voltage and currentoutput by the generator 101.

The real time voltage sensing system “A” 206 may be configured as a partof a real time control loop to control the energy output of generator101 because the sensing and controlling must be done in real time.Regarding the RMS voltage sensing and the RMS voltage sensing system “A”214, the output of generator 101 is at a high frequency (radiofrequency)which is at a different time base than the impedance change. It iscontemplated that impedance change in tissue is thermally induced; andthe tissue-thermal interaction of this sort is several orders ofmagnitude slower than the radiofrequency voltage applied through thegenerator 101.

The measured or sensed values are further processed, either by circuitryand/or a processor (not shown) in the sensor module 110 and/or by thecontrol module 102 or the hydration analyzer and controller module 102′,for deriving changes in sensed values and tissue impedance at thesurgical site “B”. Tissue impedance and changes in tissue impedance maybe determined by measuring the voltage and/or current across the tissueand/or calculating changes thereof over time, and comparing the voltageand current values to known and/or desired values associated withvarious tissue types for use by the control system 100 to driveelectrical output to achieve desired impedance and/or change inimpedance values and corresponding maxima and minima of tissue impedanceindicative of tissue hydration levels and water motility. As can beappreciated, these known and/or desired values, tissue types and rangesmay be stored in an internal look-up table, “a continuous value map” orin an external searchable memory. Commonly owned U.S. Pat. No. 6,398,779(previously mentioned), U.S. Pat. Nos. 6,203,541, 5,827,271 and U.S.Patent Application Publication No. 2003/0004510 (previously mentioned)disclose methods for measuring tissue impedance, and are incorporated byreference herein in their entirety.

Deriving tissue impedance (or other physical and electrical parameters)from real-time value(s) may provide the benefit of monitoring real-timetissue impedance and/or changes in tissue impedance. As the surgicalprocedure proceeds, it is believed that the tissue impedance fluctuatesin response to removal and restoration of liquids from the tissue at thesurgical site “B”. As the control module 102 and/or hydration analyzerand controller module 102′ monitors the tissue impedance and changes intissue impedance (or other physical and electrical parameters) thecontrol module 102 and/or hydration analyzer and controller module 102′regulates the power supply 106 and output stage 104 accordingly forachieving the desired and optimal electrosurgical effect based on thespecific portions of the impedance/conductivity readings that correlateto hydration level and direction of water motility.

Before beginning an electrosurgical procedure, an operator of theelectrosurgical instrument enters information via the user interface108. Information entered includes, for example, the type ofelectrosurgical instrument being used, the type of procedure beingperformed (i.e., desired surgical effect), the type of tissue, relevantpatient information, and a control mode setting. The control modesetting determines the amount of or type of control that the controlmodule 102 and/or hydration analyzer and controller module 102′ willprovide. As mentioned above, one or more sensors (not shown) may also beincluded to automatically provide information to the control module 102and/or hydration analyzer and controller module 102′ relating to tissuetype, initial tissue thickness, initial tissue impedance, etc.Measurement of tissue impedance to detect indications of hydrationlevels and water motility is discussed in more detail below with respectto FIG. 5.

Exemplary modes include, but are not limited to, one or a combination ofone or more of the following modes: a first mode wherein the controlmodule 102 maintains a steady selected output power, current and/orvoltage value at site “A”; a second mode wherein the control module 102and/or hydration analyzer and controller module 102′ maintains a steadyselected output power, current and/or voltage value at site “B”; a thirdmode wherein the control module 102 and/or hydration analyzer andcontroller module 102′ maintains a variable selected output power,current and/or voltage values at site “A” which is dependent upon (i.e.,a function of) time value(s) and/or sensed parameter(s) or changes insensed parameter(s) during the procedure; a fourth mode wherein thecontrol module 102 or hydration analyzer and controller module 102′maintains a variable selected output power, current and/or voltagevalues at site “B”, which is dependent upon (i.e., a function of) timevalue(s) and/or sensed parameter(s) or changes in sensed parameter(s)during the procedure.

According to the embodiments of the present disclosure, another “mode”is detecting the hydraulic conductivity (water motility) and adjustingthe rate of energy application that achieves the desired tissue effect.This is accomplished by measuring the intrinsic permeability “κ”, of thematerial [L² or m²] at the particular specific weight “γ” of the water[ML⁻²T⁻² or N m⁻³]; and the particular dynamic viscosity “μ” of water[ML⁻¹T⁻¹ or kg m⁻¹ s⁻¹] in the tissue being treated. More particularly,the absolute values of the intrinsic permeability, the specific weight,and the dynamic viscosity are not measured, but instead only changes,e.g., an increase or a decrease, in hydraulic conductivity ΔK. aremeasured.

Another mode according to the embodiments of the present disclosure istissue division which is performed having the benefit of the knowledgeof the hydraulic conductivity. This is discussed further below withrespect to FIG. 5. If hydraulic conductivity is low, hydraulic pressurewithin tissue increases more quickly with any energy deposition. Rapidrises in tissue hydraulic pressure assist in tissue division.

Functions performed on the time value(s) and sensed properties(s)include operations such as calculations and/or look-up operations usinga table or map stored by or accessible by the control module 102 and/orhydration analyzer and controller module 102′. The control module 102and/or hydration analyzer and controller module 102′ processes theselected output power, current and voltage values, such as by performingcalculations or table look up operations, to determine power controlsignal values and output control values.

The control module 102 and/or hydration analyzer and controller module102′ may determine initial settings for control signals to the powersupply 106 and the output stage 104 by using and/or processingoperator-entered data or settings, performing calculations and/oraccessing a look-up table stored by or accessible by the control module102 and/or hydration analyzer and controller module 102′. Once theelectrosurgical procedure begins, the sensors of sensor module 110 sensevarious physical and electrical properties and provide feedback to thecontrol module 102 and/or hydration analyzer and controller module 102′through the ADC 114 as needed. The control module 102 and/or hydrationanalyzer and controller module 102′ processes the feedback informationin accordance with the pre selected mode, as well as any additionaloperator-entered commands entered during the procedure. The controlmodule 102 and/or hydration analyzer and controller module 102′ thensends control information to the power supply 106 and the output stage104. The generator 101 may be provided with override controls, to allowthe operator to override the control signals provided by the controlmodule 102 and/or hydration analyzer and controller module 102′, ifneeded, e.g., by entering override commands via the user interface 108.

FIG. 3 shows a flow chart illustrating a method 300 for controllingoperation of the closed loop control system 100 during anelectrosurgical procedure in accordance with an embodiment of thepresent disclosure. At step 302, the method includes continually sensingvarious physical and electrical properties at the surgical site that arerelevant to identifying those portions of the maxima and minima oftissue conductivity/impedance readings that are specifically correlatedwith tissue hydration level and direction of water motility. At step304, the sensed properties are continually processed to calculate tissueconductivity/impedance and corresponding minima and maxima indicative oftissue hydration levels and water motility. At step 306, power supplycontrol signals are continually generated for controlling the magnitudeof the signals output by the electrosurgical generator and output stagecontrol signals are continually generated, for controlling pulseparameters of the output signals in accordance with thecontinually-processed sensed properties of maxima and minima of tissueconductivity/impedance that are indicative of tissue hydration levelsand water motility, thereby controlling the flow of water through thetissue to optimize the delivery of energy to the tissue and thusenhancing the therapeutic effect of the particular energy treatmentbeing applied. At step 308, the method includes causing tissue divisionbased upon analysis of the continually processed sensed properties.

The sensor module 110 may further include a proximity sensor for sensing(measuring) tissue thickness proximate the surgical site “B”, andgenerating a tissue thickness value. An initial tissue thickness valuemay be provided to the control module 102 as a pre-surgical parameter.Sensed real time tissue thickness values and/or changes in tissuethickness values over time (Δ[difference] thickness/Δ[difference] time)may further be provided to the control module 102 and/or hydrationanalyzer and controller module 102′ during the surgical procedure, wherethe control module 102 and/or hydration analyzer and controller module102′ modulates the electrical surgical output in accordance with thesensed real time tissue thickness values and/or changes in tissuethickness values over time as the tissue thickness values and/or changesin thickness are related to the hydration levels and water motility inthe tissue that are determined by analyzing those portions of the maximaand minima of the impedance/conductivity readings that correlate to thesame.

Accordingly, the present disclosure provides a closed loop controlsystem 100 for providing continual control of the power supply 106 andthe output stage 104 in response to “sensed” physical or electricalproperties at the surgical site and/or proximate the output stage.

In an additional embodiment according to the present disclosure and inparticular reference to FIG. 4, the control module 102 and/or hydrationanalyzer and controller 102′ is provided with two control loops, aninner loop controlled by inner loop control module 402 and an outer loopcontrolled by outer loop control module 404. The inner and outer loopcontrol modules 402, 404 are software modules executable by a processorof the control module 102 and/or hydration analyzer and controller 102′.The inner and outer loop control modules 402, 404 both receive signalsgenerated by sensor module 110.

The inner loop control module 402 controls the amount of current,voltage and/or power delivered to the tissue for controlling a variable,e.g., I, V or P, sensed at the tissue and/or calculated from sensedvalues, until a desired event occurs (a rapid dz/dt or impedance rise isachieved), e.g., an impedance value is reached that is in one embodimentin the range of about 50 ohms to about 400 ohms. The control variable iscontrolled to change during the course of the seal cycle according toimpedance value (or other sensed and/or derived values), as determinedby generator limitations (power, current, voltage) and surgicallimitations (maximum limits for application of energy to tissue).

The inner loop control module 402 continually receives real time sensedvalues, such as current I and voltage V, from the sensor module 110 andmay perform calculations on the received values for deriving additionalreal time values, such as power P and impedance Z. A desired inner loopvalue for I, V, and/or P are obtained by accessing at least one storedinner mapping of continuous values 408, look-up table or equivalent,where the inner mapping 408 may be in accordance with a function ofimpedance. In one embodiment, the inner loop control module 402 consultsthe inner mapping 408 for obtaining the desired inner loop value for theimpedance currently being sensed and derived.

An algorithm is used to compare the real time value of I, V and/or P tothe respective desired inner loop value and output an RF command to theelectrosurgical generator 101 accordingly for achieving the desiredinner loop value without exceeding the desired inner loop value, e.g.,the RF command raises the target current, voltage and/or power output bythe electrosurgical generator 101 when the real time value for I, Vand/or P is lower than the respective desired inner loop value for I, Vand/or P, and vice versa. It is contemplated that the RF commandcontrols waveform parameters of electrosurgical energy output by theelectrosurgical generator 101, including current, power, voltage, dutycycle, frequency, waveshape, etc. It is further contemplated that theinner loop is used without the outer loop for achieving the desiredtissue effect.

The outer loop control module 404, layered over the inner loop controlmodule 402, provides additional control of a variable for reaching adesired output value or effect. For example, control of the variable maymonitor/regulate the rate of change of impedance of the tissue (sensedand calculated). In different embodiments, the variables controlled mayinclude temperature, rate of change of temperature, and/or the energyinput to the tissue. Outer loop control module 404 continually receivessensed values, such as I, V and temperature T from the sensor module 110at a time “t” and performs calculations on the sensed values and in oneembodiment stored values for deriving values such as rate of change ofimpedance and/or rate of change in temperature. For example, the valuefor change in impedance (dz/dt) is obtained in accordance with:dz/dt=(Z−Z_OLD)/(t−t_OLD);  (3)Z_OLD=Z;where Z is the impedance in accordance with values measured at time t;and Z_OLD is the stored impedance in accordance with values measured ata previous time interval at time t_OLD

An outer loop desired value for the control variable is obtained byaccessing a stored outer mapping of continuous values 406, oralternatively a table or equivalent. The desired rate of changeaccording to outer mapping 406 may be steady, or may depend on the stageof the seal cycle and change over time. The tissue is in a dynamic stateduring the seal procedure, and the outer loop monitors the rate ofchange throughout the procedure to determine the degree to which thedesired rate of change is being achieved. When the control variable istemperature, a temperature map may be used for outer mapping 406 inwhich desired temperature is plotted versus time. When the controlvariable is rate of change in temperature, a rate of change intemperature map may be used for outer mapping 406 in which desiredtemperature is plotted versus time. Energy may be applied in a similarfashion, where an energy function can be calculated using equationsderived for specific tissue types or using sensed values.

An algorithm is used to compare the real time sensed/calculated value ofrate of change of impedance, temperature, rate of change of temperatureand/or energy at time “t” to the respective desired outer value at time“t” obtained from the outer mapping 406 for determining if the desiredouter value is met, and if not, for determining the ratio of thedifference between the real time value and the desired outer value tothe desired outer value. If the desired outer value is not being met,the outer loop module 406 generates a set point value which is providedto the inner loop module 402. The set point value is raised when thereal time value for rate of change of impedance, temperature and/or rateof change of temperature is lower than the respective desired outervalue for rate of change of impedance, temperature and/or rate of changeof temperature, and vice versa.

The set point value may be a ratio signal for altering the inner mapping408 by raising or lowering a plotted curve of the inner mapping 408along the y-axis. The ratio signal may be a proportional integralderivative (PID) control signal, as is known in the art. The inner loopcontrol module 402 responds instantaneously by accessing the alteredinner mapping 408 for obtaining a desired inner value from the outerloop, comparing the real time value of the control variable, generatingan RF command for achieving the desired inner value without exceedingthe desired inner value, and outputting the RF command accordingly tothe electrosurgical generator 101 for controlling voltage, currentand/or power needed for achieving a desired tissue effect.

In one embodiment, the outer loop control module 404 uses the real timevalue of rate of change of impedance, temperature, rate of change oftemperature, and/or total energy delivered to determine if a desiredouter value has been reached which indicates completion of a seal. Upondetermination of seal completion, a stop signal is generated forstopping the sealing process. Otherwise, the outer loop continues tomonitor, receive and process sensed values from the sensor module 110.

Control of I, V and/or P by the inner loop control module 402 improvessystem stability and control capabilities in low impedance ranges, e.g.,0-20 ohms, which are critical for seal initiation, particularly byavoiding a low-end impedance break point which induces oscillation andlack of system control. The outer loop control enhances the controlmodule's ability to control sealing in accordance with desired trends orevents, to change seal intensity by changing the rate of change ofimpedance, and to enhance uniform sealing of tissue, i.e., normalizetissue in terms of variability, including tissue hydration, volume andcomposition. With feedback control and continuous sensing of thetissue's condition, there is not a need to switch control variables(i.e., low/high end break points), which improves system stability asexplained above.

The control module 102 controls a module for producing resistive heatfor regulating heat applied to the tissue for achieving the desiredtissue effect instead of or in addition to controlling theelectrosurgical output stage 104 and/or the power supply 106. Thecontrol module 102 responds to sensed tissue temperature or other sensedproperties indicative of tissue temperature, accesses at least onemapping, data table or equivalent using the sensed values for obtainingdesired output current or resistivity values, and outputs a commandsignal for controlling output heat resistivity. The module for producingresistive heat may include a current source and/or a variable resistorwhich are responsive to the command signal for outputting a desiredcurrent or providing a desired resistance, respectively.

In another embodiment of the present disclosure, the control systemincludes a sensor module for sensing at least one property associatedwith a surgical site during at least one of a pre-surgical time prior toa surgical procedure, the surgical procedure and a post-surgical timefollowing the surgical procedure for generating at least one signalrelating thereto; and a control module executable on a processor forreceiving said at least one signal and processing each of said signalsusing at least one of a computer algorithm and a mapping and generatingat least one control signal in accordance with the processing, andproviding the at least one control signal to the electrosurgicalgenerator for controlling the generator. In one embodiment, theprocessing includes determining tissue type of tissue proximate thesurgical site.

In an additional embodiment, the sensor module 110 (or an additionalsensor module (not shown)) senses at least one property as apre-surgical condition, as a concurrent surgical condition and/or as apost-surgical condition. In one embodiment, the sensor module 110 sensesat least two surgical conditions (or changes in surgical conditions overtime) selected from pre-surgical, concurrent surgical and post-surgicalconditions. Pre-surgical conditions include: degree of opaqueness oftissue proximate the surgical site; moisture content level of thetissue; and/or thickness of the tissue. Concurrent conditions include:degree of opaqueness of the tissue proximate the surgical site; moisturecontent level of the tissue; thickness of the tissue; temperature of thetissue; impedance of the tissue; current across the tissue; voltageacross the tissue; power across the tissue; changes in degree ofopaqueness of the tissue; changes in moisture content level of thetissue; changes in thickness of the tissue; changes in temperature ofthe tissue; changes in impedance of the tissue; location of maxima andminima of impedance readings and specific portions of the maxima andminima that correlate to the hydration level at a specific location inthe tissue at or near the electrode-tissue interface and direction ofwater motility; changes in current across the tissue; changes in voltageacross the tissue; and changes in power across the tissue. Thepost-surgical conditions include: degree of opaqueness of tissueproximate the surgical site; moisture content level of the tissue;thickness of the tissue; temperature of the tissue; and impedance of thetissue.

Electrical cutting (rather than mechanical cutting (i.e., a knife movingthrough tissue)), referred to below as E-cutting of tissue, isfacilitated depending on whether the tissue is permeable or notpermeable. External pressure is applied on the tissue by the jaws of theelectrosurgical instrument while internal pressure, alternativelyreferred to previously as hydraulic pressure, is generated within thetissue by water that expands as its temperature increases above ambienttemperature (and its density correspondingly decreases). When the watertemperature in the tissue increases above the corresponding boilingpoint either at atmospheric pressure or under the external pressureapplied by the jaws, the water changes phase by transitioning from theliquid phase to the gaseous phase, i.e., the water changes from liquidto steam. The expansion of the water when heated to steam reduces thedensity of the water but increases the internal pressure in the tissue.The impedance of the tissue increases when steam is formed since theions are no longer present in the steam to form a conductive medium toenable the electrical current between the electrode surfaces. Since theformation of steam pockets or bubbles within the tissue increases thehydraulic pressure within the tissue, the increase in hydraulic pressurefacilitates tissue division particularly when the tissue exhibits a highreflection coefficient, as explained above.

It should be noted that the expansion of the liquid water into steam mayoccur even after the energy being applied by the electrosurgicalgenerator has been intermittently terminated due to the residual heat ofthe surrounding tissue and of the electrode or electrodes. Thus theimpedance of the tissue can increase even when energy is no longer beingapplied. In addition, during the intermittent period, water is displacedwithin the tissue. Tracking of this displacement of water providesinformation that enhances the accuracy and effectiveness of the processof tissue division being implemented by the electrosurgical system.

As the proteins within the tissue are heated, they become depolymerizedand the osmotic pressure is reduced. Such heating of the proteinschanges their hydrophilic properties.

Conversely, when energy is no longer being applied for a sufficientperiod of time, and heat is being transferred to the surroundings, thesteam pockets cannot be sustained and instead collapse, resulting in adecrease in the tissue impedance.

By sensing more precisely where in the tissue and when such expansion tosteam occurs, the e-cutting process of tissue division can be enhancedand facilitated. In addition, when tissue destruction is the desiredresult, sensing more precisely where in the tissue and when suchexpansion to steam occurs also enhances and facilitates tissuedestruction.

The foregoing principles of the present disclosure are illustrated byreference to FIG. 5. More particularly, FIG. 5 illustrates examples ofpulsed energy application of impedance versus time as shown by theimpedance at relatively lower value then quickly rising to a peak onlyto fall back to the lower value. This is one energy cycle. During the“off” cycle, a small amount of voltage remains “on” to ensure that thesensors maintain accuracy. (Sensors lose their ability to sense ifapplied voltage decays to zero). During this “off” time (example: 504 band 502 b) tissue is rehydrating and the shape of the impedance curveduring the “off” period indicates how permeable the tissue is to waterrehydration. The shape of the impedance-time trace indicates ifhysteresis occurs during the traverse of the temperature-tissueconductivity curve. Hysteresis of this nature indicates that water isnot rehydrating this tissue quickly due to low hydraulic conductivity.

Turning now specifically to FIG. 5, there is disclosed a hypotheticalcurve 500 for an ablation process of tissue impedance plotted againsttime that is analyzed in specific detail for precise location of timesof occurrence of areas in the tissue of low hydraulic conductivity(water motility) and of high hydraulic conductivity (water motility). Itis assumed that the active region of the electrode remains substantiallystationary during the ablation process, as in a bi-polar electrodeablation process. As discussed above, the present disclosure relates toa method of measuring voltage and current parameters of anelectrosurgical instrument to calculate tissue impedance. If the valuesof voltage and current are known through measurement, then the impedanceof the system can be calculated.

The impedance of all the non-tissue components of the system isgenerally constant so any fluctuations of the impedance are usuallyassociated with the dynamic changes of the treated tissue. Hence theimpedance changes hold valuable information about the dynamic change oftissue properties. One direct application of knowledge of the hydraulicconductivity (water motility) is recognition of when the tissue heldbetween the jaws of an electrosurgical instrument is ready for an energyburst that causes successful tissue division, resulting in an “E-cut,”i.e., electrical cutting (rather than mechanical cutting (i.e., knifemoving through tissue)), as mentioned above.

As is evident from FIG. 5, the impedance values often changedramatically over the duration of energy based treatments. If theimpedance is recorded over time, relative changes can be tracked. Forinstance, the impedance values often rise and fall rapidly so if thelowest values of impedance are tracked during this process, the tissuehydration can be detected. Also the response of the impedance change topower on or power off conditions can indicate how permeable the tissueis to bulk water movement or the motility of water through the tissue.Hydration levels, further supplemented by the hydraulic conductivity(the motility of water) through the tissue are good indicators of theeffectiveness of the energy based treatment because the water isnecessary for ionic conduction. As the tissue desiccates during energytreatment, the impedance rises and the application of energy becomesless efficient. Motility of the water is an indicator of the ability ofpartially desiccated tissue to become re-hydrated.

FIG. 5 illustrates characteristic impedance traces acquired during anoperation of an electrosurgical instrument, e.g., a pencil electrodeduring an ablation procedure. An advancement of the state of the artprovided by the present disclosure is the introduction of the premisethat rather than being considered system “noise” or system error, as isthe collective assessment of those skilled in the art heretofore, thereis useful information in the “W-shaped” portion during the “power off”duty cycle of the impedance trace that is indicative of the levels ofhydration in the tissue. (See for example FIG. 8 of U.S. Pat. No.6,575,969 B1).

The impedance trace in FIG. 5 is the total spatial integration of thetissue in contact with the electrode for an ablation process in whichthe active region of the electrode probe is maintained substantiallystationary with respect to the tissue, such as a bi-polar electrodesystem. Hence directivity of water displacement is not known but theassumption that displacement is in a radial direction away from the highenergy field of the active region of the electrode is a reasonableassumption.

In applications of the principles of the present disclosure to vesselsealing, more than one electrode (multiple electrode segments in a jaw),or other sensors, e.g., optical sensors, are required to determine thedirectivity of water displacement. For example, commonly owned U.S.Patent Application Publication US 2006/0064086 A1 “BIPOLAR FORCEPS WITHMULTIPLE ELECTRODE ARRAY END EFFECTOR ASSEMBLY” by Odom et al. disclosesarrays of multiple electrode segments that are each independentlypowered in each jaw member.

In the ablation process of FIG. 5, it is assumed that the bi-polarelectrode probe remains substantially stationary with respect to thetissue so that the measure of impedance in the tissue is generally onlya function of time and not of location in the tissue. Specifically, theneedle probe (the primary electrode) is positioned substantiallystationary with respect to the tissue while the secondary electrode islocated remotely from the needle probe. The bi-polar electrode probedirects electromagnetic energy to the tissue and such energy passesthrough the tissue in the vicinity of the probe and alternates indirection in the tissue as it circuitously travels through the tissuefrom one active electrode to the other active electrode in the probe.Such electromagnetic energy includes, but is not limited to,radiofrequency waves and microwave radiation. It is assumed thatmeasurable heating of the tissue is limited to a volume of tissue in thevicinity of the active region of the electrode probe and that theremaining tissue distal to the probe remains at body temperature.

[Although the example impedance versus time curve illustrated in FIG. 5applies to a bi-polar ablation electrode system, application of theprinciples of the current disclosure to a monopolar ablation electrodesystem would require that the impedance be measured both with respect tolocation as well as time. FIG. 8 of commonly owned U.S. Pat. No.6,575,969 B1 illustrates that, during a mono-polar ablation procedure, athermosurgery probe 802 is inserted into the patient's body such thatthe tip of the probe 803 is placed within the target volume 801. Theradiofrequency, laser, high frequency, or other power-generating elementis represented by component mark number 804. In the case of a highfrequency generator, a return element to a reference electrode 805 isattached to the patient's body around the shoulder region. Thisreference electrode might be a gel-pad, large area, conductive pad orother type of standard reference electrode that is used for a highfrequency generator.]

Thus, one embodiment of the present disclosure relates to a method ofdetecting the hydration and motility of water around an energy applyingmedical device such as an electrosurgical instrument. Sensing,monitoring and controlling hydration around the energy device ensuresthat the energy device is ultimately controlled so that energy isdelivered to the tissue in the most efficient manner and that theduration of the procedure is the minimum time necessary to achieve thedesired tissue effect.

In FIG. 5, precise location of times of occurrence of areas in thetissue of low water motility and of high water motility can bedetermined from the plot 500 of impedance “Z”, that may be measured inohms, versus time “t”. The plot 500 begins with an initial impedance“Z₁” and includes a plurality of intermittent crests or peaks 502 a, 502b, 502 c, 502 d, 502 e, 502 f, 502 g, and 502 h, i.e., the intermittentmaxima, and a corresponding plurality of intermittent troughs or valleys504 a, 504 b, 504 c, 504 d, 504 e, 504 f and 504 g, i.e., theintermittent minima. A curve 506 is drawn that envelopes the minima 504a through 504 g.

Following the initial impedance “Z₁”, which occurs at body temperature,even though energy is applied from the electrosurgical generator 101such that the local temperature of the tissue rises to about 80 degreesCelsius (° C.), the tissue impedance Z drops to a first minimum at 504a, During the next phase, from minimum 504 a to first maximum 502 a,energy from the electrosurgical generator 101 is terminated at firstmaximum or peak 502 a. During the approach to peak 502 a, thetemperature of the water within the tissue has increased to the boilingpoint, e.g., 100° C., such that a phase change from liquid to vaporoccurs, i.e., a steam pocket or bubble has formed within the tissue atpeak 502 a. As the temperature increases towards the boiling point, theenergy continues to be applied and the impedance increases. The phasechange to the vapor state causes an increase in hydraulic conductivity.During the decrease in impedance Z from first peak 502 a to secondminimum 504 b, energy remains terminated or turned off, the steam bubblecollapses. Osmotic pressure draws water in to fill the void caused bythe collapse of the steam bubble. Thus, the increase in water contentcauses the hydraulic conductivity to increase and the impedance todecrease. Energy is re-applied at second minimum 504 b so that thehydraulic conductivity decreases and the impedance increases untilsecond maximum or peak 502 b. Again, the temperature of the water withinthe tissue has increased to the boiling point such that a phase changeoccurs at second maximum or peak 502 b.

Upon reaching the second maximum or peak 502 b, energy is againterminated and the tissue re-hydrates until a first curvature changepoint 5081 occurs wherein a marked change in slope of the impedancecurve Z versus t occurs such that the decay in impedance decreases andboth the hydraulic conductivity and the impedance remain relativelyconstant. The energy remains terminated until the impedance Z begins todecrease again, and conversely the hydraulic conductivity begins toincrease again, at a second curvature change point 5082 at which timethe energy is re-applied, the impedance Z decreases for a short timeuntil a third minimum 504 c is reached.

During the decrease in impedance from second maximum 502 b to thirdminimum 504 c, the formation of first and second curvature change points5081 and 5082, respectively, is an indication that tissue hysteresis hasoccurred. That is, the tissue has “remembered” the prior application ofenergy and a change in tissue properties has occurred. The change intissue properties results in reduced hydraulic conductivity during thetransition from second maximum 502 b to third minimum 504 c as comparedto the transition from first maximum 502 a to second minimum 504 b, asindicated by the first and second curvature change points 5081 and 5082,respectively. The motility of the water within the tissue tends todecrease, i.e., the flow of water is inhibited within the tissue.

As application of the energy continues and the tissue dehydrates, theimpedance Z increases again sharply until a third maximum or peak 502 cis reached. In a similar manner as during the approach to maximum orpeak 502 b, during the approach to maximum or peak 502 c, thetemperature of the water within the tissue has increased to the boilingpoint such that a phase change occurs at third maximum or peak 502 c.

Energy is again terminated at third peak 502 c. Thereby a first modifiedV-shape 5241 is formed in the data plot 500 that is indicative of aregion in the tissue wherein there is low motility of water.

The rate of tissue rehydration changes after third peak 502 c ascompared to after second peak 502 b. More particularly, the steam bubbleor pocket that was formed during the ascent to third peak 502 c againcollapses and the osmotic pressure causes the water to re-enter thetissue undergoing the ablation process. The rate of increase ofhydraulic conductivity decreases until a change in the rate of decreaseof impedance Z again occurs at first curvature change point 5101. Ascompared to the relatively constant impedance Z versus time t occurringafter first curvature change point 5081, after first curvature changepoint 5101, the motility and the hydraulic conductivity of the water inthe tissue increase slightly such that the impedance Z decreasesslightly until reaching second curvature change point 5102, at whichpoint energy is re-applied until the impedance Z decreases to fourthminimum 504 d. Again, the formation of first and second curvature changepoints 5101 and 5102, respectively, is indicative of tissue hysteresisand thus a change in the properties of the tissue due to the ablationprocess.

Again, as application of the energy continues and the tissue dehydrates,the impedance Z increases again sharply until a fourth maximum or peak502 d is reached. Energy is again terminated at fourth peak 502 d.Thereby a second modified V-shape 5242 is formed in the data plot 500that is also indicative of a region in the tissue wherein there is lowmotility of water.

The rate of tissue rehydration changes again after fourth peak 502 d ascompared to after third peak 502 c. More particularly, the rate ofincrease of hydraulic conductivity decreases until a change in the rateof decrease of impedance Z again occurs at first curvature change point5121. As compared to the impedance Z versus time t occurring after firstcurvature change point 5101, after first curvature change point 5121,the motility and the hydraulic conductivity of the water in the tissueincrease at a more pronounced rate such that the impedance Z decreasesat a more pronounced rate until reaching second curvature change point5122, at which point energy is re-applied until the impedance Zdecreases to fifth minimum 504 e.

The application of energy continues, the tissue again dehydrates, andthe impedance Z increases again sharply until a fifth maximum or peak502 e is reached. Energy is again terminated at fifth peak 502 e.Thereby a third modified V-shape 5243 is formed in the data plot 500that is again indicative of a region in the tissue wherein there is lowmotility of water.

In contrast to the second modified V-shape 5242, the third modifiedV-shape 5243 indicates a higher level of hydraulic conductivity sincethe impedance Z decreases at a faster rate prior to fifth minimum 504 eas compared to the second modified V-shape 5242 prior to fourth minimum504 d.

In the example illustrated in FIG. 5, following the termination of theenergy at fifth maximum 502 e, again the tissue impedance decreases butinstead of reaching a sharp minimum such as 504 b, 504 c, 504 d and 504e, a first blunt minimum 514 a is formed and the tissue impedancesubsequently increases while the energy continues to be terminated untila low impedance maximum 514 is formed, During the time period betweenthe first blunt minimum 514 a and the low impedance maximum 514, thetissue becomes less permeable and the amount of water entering thetissue decreases. Correspondingly, the hydraulic conductivity of thetissue decreases. The reduced permeability increases the waterreflection coefficient of the tissue.

At low impedance maximum 514, the energy is again applied and theimpedance decreases for a brief period until a second blunt minimum 514b is formed. As the application of energy is continued, the impedanceincreases, while conversely, the hydraulic conductivity decreases, tosixth maximum or peak 502 f. The indication of virtual equality of theimpedance levels Z at first blunt minimum 514 a and second blunt minimum514 b is evidence that the displacement of water within the tissue issubstantially equal in both directions. In addition, when the bluntminima in a W-shape are substantially equal, the hydration level and thetissue permeability are equal. The low impedance peak 514 between thetwo blunt minima 514 a and 514 b forms a W-shape 5261 that is indicativeof a time period during which there is high motility of water in thetissue as compared to the time periods of the modified V-shapes 5241,5242 and 5243. At the time of occurrence of the blunt minima 514 a and514 b, the tissue membranes have a porosity and a reflectancecoefficient that are optimal for the occurrence of tissue division uponfurther application of electrical energy from the electrosurgicalgenerator. The virtual equality of the blunt minima 514 a and 514 b isindicative of a lack of tissue hysteresis during this time period and asa result no change in tissue impedance. Thus the occurrence of the bluntminima 514 a and 514 b presents in effect almost a “last opportunity” toeffect tissue division using the electrical energy from theelectrosurgical generator.

From the initial impedance Z1 to the impedance at sixth maximum 502 f,the plot 500 of impedance Z versus time t has followed anelectrosurgical ablation procedure according to the prior art in whichtissue division was not intended to occur. In the prior art ablationprocedure, the significance of the modified V-shapes 5241, 5242 and 5243and of the W-shape 5261 with respect to tissue hydration and dehydrationare disregarded with respect to the control of energy received from theelectrosurgical generator 101.

According to the present disclosure, the detailed analysis of thesignificance of the modified V-shapes 5241, 5242 and 5243 and of theW-shape 5261 with respect to tissue hydration and dehydration areincorporated into a method of initiating tissue division usingelectrical energy during an ablation process, as opposed to a separatestep being required following the ablation process of mechanical cuttingof the tissue. More particularly, following the occurrence of the secondblunt minimum 514 b, the energy from the electrosurgical generator 101is increased such that the impedance Z increases along the dashed line5281 to a level at maximum 502 f that is greater than the impedance ofthe tissue at sixth maximum 502 f so as to be sufficient to cause tissuedivision.

In lieu of increasing the energy from the electrosurgical generator 101upon achieving sixth maximum 502 f, in order to verify that a timeperiod of relatively high tissue hydration has occurred, the surgeon mayinstead choose to continue along the curve 500 by terminating the energyat sixth maximum 502 f to allow the tissue impedance Z to decrease toapproach a first blunt minimum 518 a followed by a low impedance peak518 at which the energy is again applied and the impedance decreases fora brief period until a second blunt minimum 518 b is formed. Again, asthe application of energy is continued, the impedance increases, whileconversely, the hydraulic conductivity decreases, to seventh maximum orpeak 502 g. The low impedance peak 518 between the two blunt minima 518a and 518 b forms a second W-shape 5262 that is also indicative of atime period during which there is high motility of water in the tissueas compared to the time periods of the modified V-shapes 5241, 5242 and5243. However, the difference in the level of impedance Z between firstblunt minimum 518 a and second blunt minimum 518 b, i.e., the impedanceZ at second blunt minimum 518 b is significantly greater than theimpedance Z at first blunt minimum 518 a, and thus is indicative of theoccurrence of tissue hysteresis and a change in properties of thetissue, in contrast to the lack of tissue hysteresis and change intissue properties associated with blunt minima 514 a and 514 b.

In the electrosurgical ablation method of the prior art, the energy isagain terminated at seventh maximum 502 g and the impedance Z decreasesto a sharp minimum 504 f. Energy is again applied and the impedanceincreases to eighth maximum 502 h. Upon reaching the eighth maximum 502h, energy is again terminated and the impedance Z decreases to a sharpminimum 504 g. A final application of energy following minimum 504 gresults in an increase in impedance to ninth maximum 502 i at whichpoint the ablation procedure is terminated without causing tissuedivision.

In contrast, in the electrosurgical ablation method according to thepresent disclosure, in lieu of terminating the energy at seventh maximum502 g, having confirmed the presence of the first W-shape 5261, thesurgeon may manually, or the software controlling the electrosurgicalgenerator 101 is programmed such that, following the occurrence of thesecond blunt minimum 518 b, the energy from the electrosurgicalgenerator 101 is increased such that the impedance Z increases along thedashed line 5282 to a level at maximum 502 g′ that is greater than theimpedance of the tissue at seventh maximum 502 g so as to be sufficientto cause tissue division.

During the transition from blunt minimum 514 a to low impedance peak514, and from blunt minimum 518 a to low impedance peak 518, the energyapplied forms a steam pocket that causes an increase in impedance. Theimpedance decreases from low impedance peak 514 to blunt minimum 514 band from low impedance peak 518 to blunt minimum 518 b after the energyis terminated following the occurrence of the low impedance peaks 514and 518.

As the tissue begins to desiccate in the region following the firstW-shape 5261, the magnitude of the difference between the second lowimpedance peak 518 and the corresponding first blunt minimum 518 a ascompared to the difference between the second low impedance peak 518 andcorresponding second blunt minimum 518 b begins to decrease as comparedto the magnitude of the difference between the first low impedance peak514 and the corresponding substantially equal blunt minima 514 a and 514b. It can be seen also from the general trend of the enveloping curve506 that the impedance Z also begins to increase following the firstW-shaped region 5261 since the quantity of water present in the tissueis no longer sufficient to facilitate hydraulic conductivity.

Referring also to FIG. 4, during operation, via the electrosurgicalgenerator 101 (see FIG. 4), of the electrosurgical instrument performingthe ablation, conductivity, which is the inverse of impedance, ismeasured (via one or more sensor modules 110) to provide tissueproperties and the degree of hydration and motility of water in thetissue. The first derivative of the conductivity is calculated by theouter loop control module 404 to establish a setpoint value to becommunicated to the inner loop control module 402 to control theelectrosurgical generator 101. The maxima and minima of theconductivity/impedance curve can be determined by setting the firstderivative of the impedance “Z” as a function of time equal to zero,i.e., “dZ/dt=0” control of the impedance “Z” enables control of thehydration level in the tissue and enables also taking advantage of timesin which the reflection coefficient is of a sufficient magnitude tofacilitate and enhance tissue division and tissue destruction, when thelatter is desired. Conversely, control of the hydration level in thetissue enables proper control of the impedance “Z”.

Thus, controlling the application of electrosurgical energy to thesurgical site optimizes energy delivery based on the hydration leveland/or the water motility and/or the reflection coefficient in thetissue at the surgical site. The electrosurgical energy applied to thesurgical site is a function of the hydration level and/or water motilitywhich in turn are identified via specific portions of the readings ofmaxima and minima of tissue impedance readings.

It should be noted that detecting the generally modified “V-shape” and“W-shape” parameters does not necessarily require modifying theimpedance trace that is illustrated in FIG. 5 prior to the increasedapplication of energy exhibited by dashed lines 5281 and 5282. However,additional filtering or processing of the impedance trace may enhancedetection of the “W-shape” parameters by detailed data analysisperformed by the processor software.

At least one property sensed during the post-surgical condition may beindicative of the quality of a tissue seal formed during the surgicalprocedure. In one embodiment, the sensor module 110 includes a lightdetector for detecting light generated by a light source and transmittedthrough (or reflected from) the tissue proximate the surgical site. Aproximity sensor having sensing elements placed at opposite surfaces ofthe tissue may also be included for sensing the distance between theelements which is indicative of the tissue thickness.

From the foregoing description and referring again to FIGS. 1-5, it canbe appreciated that the present disclosure relates to a method forperforming an electrosurgical procedure at a surgical site on a patient.Referring particularly to FIG. 3, the method includes the steps ofcontinually sensing electrical and physical properties proximate thesurgical site, i.e., steps 302 and 304, in which the step of continuallysensing properties includes: acquiring readings of tissue electricalimpedance with respect to time at the surgical site; identifying theminima and maxima of the impedance readings with respect to time; andcorrelating at least one of the minima and the maxima of the impedancereadings with at least one of hydration level and hydraulic conductivityin the tissue at the surgical site, i.e., steps 304 and 306. The methodalso includes controlling the application of electrosurgical energy tothe surgical site to vary energy delivery based on the step ofcorrelating at least one of the minima and the maxima of the impedancereadings with at least one of the hydration level and the hydraulicconductivity in the tissue at the surgical site, i.e., step 306. Thoseskilled in the art will recognize that the foregoing steps may beimplemented by the closed loop control system 100 for use withelectrosurgical generator 101 (see FIGS. 1, 2 and 4).

As explained above with respect to FIG. 4, the method may include thestep of calculating the inverse of the tissue electrical impedance Zreadings to determine readings of tissue electrical conductivity. Thestep of varying pulse parameters is performed by controlling at leastone of the hydration level and water motility by controlling tissueconductivity levels.

As illustrated in FIG. 5, the electrosurgical procedure is an ablationprocess. The step of correlating at least one of the minima and themaxima of the impedance readings with at least one of hydration leveland hydraulic conductivity in the tissue at the surgical site isperformed by identifying at least one minimum, e.g., minimum 504 c, thatis preceded by a change in slope, e.g., the interval between first andsecond curvature change points 5081 and 5082, respectively, representinga decrease in the rate of descent of impedance from a prior maximum,e.g., maximum 502 b, to the next minimum, e.g., minimum 504 c, formingthereby at least one generally modified V-shaped plot, e.g., generallymodified V-shaped plot 5241, indicative of a change in slope in theV-shape.

In one embodiment, step 306 of correlating at least one of the minimaand the maxima of the impedance readings with at least one of hydrationlevel and hydraulic conductivity in the tissue at the surgical site maybe performed by identifying at least one maximum that is formed of a lowimpedance peak, e.g., maximum 514, between two blunt minima, e.g., firstblunt minimum 514 a and second blunt minimum 514 b, to form at least onegenerally W-shaped plot 5261.

In one embodiment, step 306 of correlating at least one of the minimaand the maxima of the impedance readings with at least one of hydrationlevel and hydraulic conductivity in the tissue at the surgical site mayalso be performed by identifying at least one minimum, e.g., minimum 504c, that is preceded by a change in slope, e.g., the interval betweenfirst and second curvature change points 5081 and 5082, respectively,representing a decrease in the rate of descent of impedance from a priormaximum, e.g., maximum 502 b, to the next minimum, e.g., minimum 504 c,forming thereby at least one generally modified V-shaped plot, e.g.,generally modified V-shaped plot 5241, indicative of a change in slopein the V-shape and by identifying at least one maximum that is formed ofa low impedance peak, e.g., maximum 514, between two blunt minima, e.g.,first blunt minimum 514 a and second blunt minimum 514 b, to form atleast one generally W-shaped plot 5261.

Additionally, the method may further include the step of concluding thatthe occurrence of the at least one generally W-shaped plot, e.g.,W-shaped plot 5261, represents a time period of high Water motility inthe patient tissue as compared to the at least one generally modifiedV-shaped plot, e.g., generally modified V-shaped plot 5241.

Also, the method may further include the step of concluding that theoccurrence of the at least one generally modified V-shaped plot, e.g.,generally modified V-shaped plot 5241, represents a time period whereinthere is low hydraulic conductivity of water in the tissue at thesurgical site as compared to the time period of the at least onegenerally W-shaped plot, e.g., W-shaped plot 5261.

The method may further include the step of comparing the difference inimpedance between at least one low impedance peak, e.g., peak 514, tothe impedance of the corresponding two blunt minima, e.g., blunt minima514 a and 514 b, of at least one generally W-shaped plot, e.g., W-shapedplot 5261, to the difference in impedance between at least one lowimpedance peak, e.g., peak 518, to the impedance of the correspondingtwo blunt minima, e.g., blunt minima 518 a and 518 b, of at leastanother generally W-shaped plot, e.g., W-shaped plot 5262.

The method may further include the step of concluding that the timeperiod of the at least one generally W-shaped plot, e.g., W-shaped plot5261, represents a time of high motility of water in the tissue ascompared to the time period of the at least another generally W-shapedplot, e.g., W-shaped plot 5262.

The surgeon may implement the method by decreasing application ofelectrosurgical energy to the surgical site upon identifying the atleast one generally modified V-shaped plot, e.g., V-shaped plot 5241 or5242 or 5243. The surgeon may also implement the method by increasingapplication of electrosurgical energy to the surgical site to a levelsufficient to initiate tissue division upon identifying the at least onegenerally W-shaped plot, e.g. W-shaped plot 5261, such that theimpedance Z increases along the dashed line 5281 to a level at maximum502 f′ that is greater than the impedance of the tissue at sixth maximum502 f so as to be sufficient to cause tissue division, e.g., step 308 ofFIG. 3.

As discussed above, In lieu of increasing the energy from theelectrosurgical generator 101 upon achieving sixth maximum 502 f, inorder to verify that a time period of relatively high tissue hydrationhas occurred, the surgeon may instead choose to continue along the curve500 by terminating the energy at sixth maximum 502 f to allow the tissueimpedance Z to decrease to approach first blunt minimum 518 a followedby a low impedance peak 518 at which the energy is again applied and theimpedance decreases for a brief period until a second blunt minimum 518b is formed.

In lieu of terminating the energy at seventh maximum 502 g, havingconfirmed the presence of the first W-shape 5261, the surgeon maymanually, or the software controlling the electrosurgical generator 101is programmed such that, following the occurrence of the second bluntminimum 518 b, the energy from the electrosurgical generator 101 isincreased such that the impedance Z increases along the dashed line 5282to a level at maximum 502 g′ that is greater than the impedance of thetissue at seventh maximum 502 g so as to be sufficient to cause tissuedivision, e.g., step 308 of FIG. 3.

Although this disclosure has been described with respect to variousembodiments, it will be readily apparent to those having ordinary skillin the art to which it appertains that changes and modifications may bemade thereto without departing from the spirit or scope of thedisclosure. For example, it is contemplated that the control module 102and/or hydration analyzer and controller 102′ may include circuitry andother hardware, rather than, or in combination with, programmableinstructions executed by a microprocessor for processing the sensedvalues and determining the control signals to be sent to the powersupply 106 and the output stage 104.

While several embodiments of the disclosure have been shown in thedrawings, it is not intended that the disclosure be limited thereto, asit is intended that the disclosures be as broad in scope as the art willallow and that the specification be read likewise. Therefore, the abovedescription should not be construed as limiting, but merely asexemplifications of particular embodiments.

What is claimed is:
 1. A method for performing an electrosurgicalprocedure at a surgical site on a patient, the method comprising:continually sensing electrical and physical properties proximate thesurgical site by: acquiring data readings of an electrical impedance oftissue with respect to time at the surgical site; identifying portionsof the data readings of the electrical impedance of the tissue forminima and maxima; and determining a hydration level and a direction ofwater motility in the tissue at the surgical site; controllingapplication of electrosurgical energy to the tissue at the surgical siteto vary one or more parameters of energy delivered based on thehydration level and the direction of water motility in the tissue at thesurgical site; and terminating application of electrosurgical energy ata first minimum of the electrical impedance of the tissue.
 2. The methodof claim 1, further comprising: calculating the inverse of the datareadings of the electrical impedance of the tissue to determine datareadings of an electrical conductivity of the tissue.
 3. The method ofclaim 1, wherein the controlling application of the electrosurgicalenergy includes varying pulse parameters based on either one or both ofthe hydration level and the direction of water motility by sensingtissue conductivity levels.
 4. The method of claim 1, wherein theelectrosurgical procedure is an ablation process, and wherein theidentifying further includes: identifying at least one minimum ofelectrical impedance of the tissue that is preceded by a change in sloperepresenting a decrease in a rate of descent of the electrical impedancefrom a prior maximum of electrical impedance of the tissue to the atleast one minimum of electrical impedance of the tissue forming therebyat least one generally modified V-shaped plot.
 5. The method of claim 4,further comprising: decreasing the application of electrosurgical energyto the tissue at the surgical site upon identifying the at least onegenerally modified V-shaped plot.
 6. The method of claim 1, wherein theelectrosurgical procedure is an ablation process, and wherein theidentifying further includes: identifying at least one maximum ofelectrical impedance of the tissue that is formed of a low impedancepeak between a first blunt minimum and a second blunt minimum, whereinthe at least one maximum of electrical impedance of the tissue, thefirst blunt minimum, and the second blunt minimum form at least onegenerally W-shaped plot.
 7. The method of claim 6, further comprising:increasing the application of electrosurgical energy to the tissue atthe surgical site to a level sufficient to initiate tissue division uponidentifying the at least one generally W-shaped plot.
 8. The method ofclaim 1, wherein the electrosurgical procedure is an ablation process,and wherein the identifying further includes: identifying at least oneminimum of electrical impedance of the tissue that is preceded by achange in slope representing a decrease in a rate of descent of theelectrical impedance of tissue from a prior maximum of electricalimpedance of the tissue to the at least one minimum of electricalimpedance of the tissue forming thereby at least one generally modifiedV-shaped plot; and identifying at least one maximum that is formed of alow impedance peak between a first blunt minimum and a second bluntminimum, wherein the at least one maximum, the first blunt minimum, andthe second blunt minimum form at least one generally W-shaped plot. 9.The method of claim 8, further comprising: concluding that an occurrenceof the at least one generally W-shaped plot is indicative of a highwater motility in the tissue at the surgical site.
 10. The method ofclaim 8, further comprising: concluding that an occurrence of the atleast one generally modified V-shaped plot is indicative of a lowhydraulic conductivity of water in the tissue at the surgical site. 11.A method for performing an electrosurgical procedure at a surgical siteon a patient, the method comprising: continually sensing electrical andphysical properties proximate the surgical site by: acquiring datareadings of an electrical impedance of tissue with respect to time atthe surgical site; identifying portions of the data readings of theelectrical impedance of the tissue for minima and maxima; anddetermining a direction of water motility in the tissue at the surgicalsite; terminating application of electrosurgical energy to the tissuebased on the portions of the data readings of the electrical impedanceof the tissue and reapplying electrosurgical energy after terminatingapplication of electrosurgical energy; and controlling application ofelectrosurgical energy to the tissue at the surgical site to vary one ormore parameters of energy delivered based on the portions of the datareadings for the minima and the maxima of the tissue electricalimpedance indicative of the direction of water motility in the tissue atthe surgical site.
 12. The method of claim 11, further comprising:continually outputting electrical signals indicative of the hydrationlevel and the direction of water motility in the tissue at the surgicalsite.
 13. The method of claim 12, wherein the controlling of theapplication of electrosurgical energy to the tissue at the surgical siteincludes continually receiving electrical signals to determine thehydration level and the direction of water motility in the tissue at thesurgical site and varying one or more parameters of energy deliveredbased on one or more of the electrical signals to determine thehydration level and the direction of water motility in tissue at thesurgical site.
 14. A method for performing an electrosurgical procedureat a surgical site on a patient, the method comprising: continuallysensing electrical and physical properties proximate the surgical siteby: acquiring data readings of an electrical impedance of tissue withrespect to time at the surgical site; performing either one or both ofgraphical analysis and numerical analysis to identify portions of thedata readings of the electrical impedance of tissue for minima andmaxima of the tissue electrical impedance and determining a direction ofresistance to water motility in tissue at the surgical site, includingeither one or both of: identifying at least one minimum of theelectrical impedance of tissue that is preceded by a change in sloperepresenting a decrease in a rate of descent of the electrical impedancefrom a prior maximum of the electrical impedance of tissue to the atleast one minimum forming thereby at least one generally modifiedV-shaped plot to determine a low motility of water in the tissue at thesurgical site; and identifying at least one maximum of the electricalimpedance of tissue that is formed of a low impedance peak between afirst blunt minimum and a second blunt minimum, wherein the at least onemaximum of the electrical impedance of tissue, the first blunt minimum,and the second blunt minimum form at least one generally W-shaped plotto determine a high motility of water in the tissue at the surgicalsite; and controlling application of electrosurgical energy to thetissue at the surgical site to vary one or more parameters of energydelivered based on the direction of resistance to water motility in thetissue at the surgical site including terminating application ofelectrosurgical energy in response to reaching a minimum of theelectrical impedance of the tissue and reapplying application ofelectrosurgical energy in response to a maximum of the electricalimpedance of the tissue.